Article pubs.acs.org/JPCA
Cyclic Constraints on Conformational Flexibility in γ‑Peptides: Conformation Specific IR and UV Spectroscopy Patrick S. Walsh,† Ryoji Kusaka,†,§ Evan G. Buchanan,† William H. James III,†,# Brian F. Fisher,‡ Samuel H. Gellman,‡ and Timothy S. Zwier*,† †
Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907 United States Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 United States § Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan. ‡
S Supporting Information *
ABSTRACT: Single-conformation spectroscopy has been used to study two cyclically constrained and capped γ-peptides: Ac-γACHC-NHBn (hereafter γACHC, Figure 1a), and Ac-γACHC-γACHC-NHBn (γγACHC, Figure 1b), under jet-cooled conditions in the gas phase. The γ-peptide backbone in both molecules contains a cyclohexane ring incorporated across each Cβ-Cγ bond and an ethyl group at each Cα. This substitution pattern was designed to stabilize a (g+, g+) torsion angle sequence across the Cα−Cβ− Cγ segment of each γ-amino acid residue. Resonant two-photon ionization (R2PI), infrared−ultraviolet hole-burning (IR−UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been used to probe the single-conformation spectroscopy of these molecules. In both γACHC and γγACHC, all population is funneled into a single conformation. With RIDIR spectra in the NH stretch (3200−3500 cm−1) and amide I/ II regions (1400−1800 cm−1), in conjunction with theoretical predictions, assignments have been made for the conformations observed in the molecular beam. γACHC forms a single nearest-neighbor C9 hydrogen-bonded ring whereas γγACHC takes up a nextnearest-neighbor C14 hydrogen-bonded structure. The gas-phase C14 conformation represents the beginning of a 2.614-helix, suggesting that the constraints imposed on the γ-peptide backbone by the ACHC and ethyl groups already impose this preference in the gas-phase di-γ-peptide, in which only a single C14 H-bond is possible, constituting one full turn of the helix. A similar conformational preference was previously documented in crystal structures and NMR analysis of longer γ-peptide oligomers containing the γACHC subunit [Guo, L., et al. Angew. Chem. Int. Ed. 2011, 50, 5843−5846]. In the gas phase, the γACHCH2O complex was also observed and spectroscopically interrogated in the molecular beam. Here, the monosolvated γACHC retains the C9 hydrogen bond observed in the bare molecule, with the water acting as a bridge between the C-terminal carbonyl and the π-cloud of the UV chromophore. This is in contrast to the unconstrained γ-peptide-H2O complex, which incorporates H2O into both C9 and amide-stacked conformations. As Figure 1d illustrates, the conformation of a γ residue is determined by four dihedral angles. Two are analogous to the Ramachandran angles used to determine α-amino acid residue conformation (ϕ, ψ), and the other two arise from the extended γ residue backbone (θ, ζ). This increase in backbone complexity in a γ residue relative to an α residue might be anticipated to result in a wider range of competing low-energy structures, which could lead to a greater diversity in folding preferences for γ-peptides relative to α-peptides. The presence of three positions for substitution within each γ residue also offers a richer palette of side chain-based strategies to control γ residue conformational propensity relative to the options available for α residues. Several previous studies focused attention on γ-peptides with side chains similar to those in α-
I. INTRODUCTION Proteins are composed of α-amino acid residues, in which one carbon atom (Cα) separates adjacent amide groups. There has been increasing interest in extending the set of protein-like oligoamides to include other building blocks, including β-amino acid residues (two carbons between amide groups) and γ-amino acid residues (three carbons between amide groups). These protein-inspired oligomers can also contain heterogeneous backbones, in which α, β, and/or γ residues are combined. These synthetic foldamers are designed to adopt secondary structures comparable to those found in biopolymers (e.g., helices, sheets, reverse turns), and to display properties beyond those available to conventional peptides and proteins, including resistance to enzymatic degradation and greater conformational stability.1−6 Among the structural elements that have benefited most from this development are helices, which can be formed with a variety of helical pitches, radii, and side-chain architecture. © 2013 American Chemical Society
Received: August 31, 2013 Revised: October 17, 2013 Published: October 22, 2013 12350
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Figure 1. Structures of (a) Ac-γACHC-NHBn and (b) Ac-(γACHC)2-NHBn with possible intramolecular H-bonds and ring sizes included. Hydrogen bonds shown in blue go from the C- to the N-terminus whereas those in red are formed from the N- to the C-terminus. (c) Carbon labeling scheme for the carbons in the backbone and (d) the naming convention for the dihedral angles. The hydrogen-bonded rings are labeled as Cn, where n indicated the number of atoms contained in the ring.
peptides.7−13 Alternatively, ring-based constraints have been built into γ residues that are intended to promote secondary structure specificity.14 One promising substitution pattern combines a cyclohexane ring that incorporates the Cβ−Cγ backbone bond with ethyl substitution at Cα. These substituents create a total of three chiral centers, and an expeditious synthetic route is availabile for γACHC in which all three centers have S configuration or all three have R configuration. This substitution pattern leads to a propensity for a g+, g+ (θ, ζ) torsion angle sequence, which imparts overall curvature across the γ residue. Several studies have been carried out in condensed phases to determine the conformations adopted by short γ-peptides, and to discover how conformational behavior evolves as the length of the γ-peptide chain grows. One of the most wellcharacterized γ-peptide secondary structures, the 14-helix, contains 14-atom, i, i + 3 CO···H−N H-bonds.7,8,14−17 The recent work of Guo et al. has provided X-ray crystal structures and NMR spectral signatures (NOEs) that arise from the γ-peptide 14-helix, which has 2.6 residues per turn (2.614).14 The 14-helix has been detected in nonpolar solution for tetramers and longer oligomers.7,8 By comparison, short gabapentin-containing γ-peptides, disubstituted at Cβ, prefer C9 “nearest-neighbor” H-bonds in condensed phases.14,18,19 Investigations of the conformational preferences of short peptidic foldamers in the gas phase offers a perspective complementary to that available from condensed-phase studies, which can deepen insight regarding the delicate balance of forces that operate within these novel backbones. When solvent is removed, the gas-phase studies reveal inherent conformational preferences. Such studies also offer a direct connection with high-level ab initio computational studies, and can
contribute to the development of molecular mechanics force fields that correctly simulate non-natural foldamer subunits. As a part of our ongoing studies of the inherent conformational preferences of synthetic foldamers,20−23 we have recently carried out a systematic investigation of the preferred conformations of a series of gas-phase γ-peptides cooled in a supersonic expansion.24−27 Our initial studies of the prototypical capped diamide Ac-γ2-hPhe-NHMe (Figure 2a) showed that the majority of the population formed C9 Hbonds.24 However, about 20% of the population existed in an
Figure 2. (a) Chemical structure of Ac-γ2-hPhe-NHMe and (b) a typical amide-stacked conformation of Ac-γ2-hPhe-NHMe.24 12351
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generated and skimmed ∼2 cm downstream to form a molecular beam. The molecular beam then enters the extraction region of a time-of-flight mass spectrometer (TOF, Wiley-McClaren) where the molecules of interest are probed via resonant two-photon ionization through the S 0 −S 1 transition of the benzyl cap. The γACHC-H2O complex was formed with trace H2O present in the sample or gas handling lines. Detailed explanations of the various single- and doubleresonance schemes used in this work have also been described previously, and are briefly summarized here.30−32 The UV spectrum in the S0−S1 origin region was recorded using onecolor resonant two-photon ionization (R2PI) where tunable UV light was generated using the doubled output of a Nd:YAG pumped tunable dye laser (Radiant Dyes NarrowScan, Coumarin 540A) with typical outputs of
